During the past several decades major changes have occurred in the handling of seagoing cargo between ships and the dock. The most noteworthy development has been the evolution and utilization of container ships and container cranes. Container ships allow the handling of cargo packed in standardized rectangular containers which allow their systematic movement by appropriately configured equipment from location to location and the stacking of these containers for storage and further handling. A container crane is a large, dockside, gantry crane that can move lengthwise on railroad rails along the dock. This type of crane typically has a large, horizontal trolley beam or girder which is vertically and permanently fixed at some particular elevation and extends over a ship moored alongside a dock. Along this trolley girder is a moveable carriage or trolley which rides on the beam. Wire ropes on the trolley suspend a lift beam, also known as a spreader, which is designed to engage the corners of a cargo container by a device called a twist lock. One twist lock type of mechanism is described in a Loomis et al. U.S. Pat. No. 3,749,438. The trolley is operable along the trolley girder, part of which is called a boom if it can be pivotally raised or lowered about a hinge point, for carrying a cargo container through the entire horizontal distance between the dock and the ship. Vertical hoisting or lowering of a container by the trolley along this horizontal path is also possible which allows clearance of any obstacles in the path.
This typical single hoisting trolley container crane has several shortcomings, among which is the fact that the permanently fixed trolley girder must be located high enough to be able to work above many different sizes of vessels. Because the girder must be positioned and fixed at such a high elevation, there is an increase in operator parallax and in the distance to where the spreader is to be positioned for attachment to a container. Although this type of single hoist crane can be designed for horizontal trolley travel at increasingly higher speeds in an attempt to realize better operating cycle times, the fact that the container load is usually suspended some distance below the trolley by wires creates undesirable sway characteristics which oppose a decrease in the overall cycle time.
In recognizing the limitations of single hoist cranes, other prior art proposals have included the utilization of a dual hoist type of crane which breaks the operating cycle into two separate phases in a manner very similar to what is known by fire fighters as a bucket brigade. A bucket brigade allows the faster passing of pails of water from person to person, as opposed to persons running individually from the source of the water to the fire site. Examples of dockside container cranes using dual hoist principles are described in the following U.S. Pat. Nos.: 3,812,987 (Watatani); 4,046,265 (Wormmeester et al.); 4,106,639 (Montgomery et al.); 4,172,685 (Nabeshima et al.); 4,293,077 (Makimo); and 4,599,027 (Knapp).
These patents describe various types of cranes, some utilizing a stationery intermediate platform with a second hoisting trolley, others using elevating intermediate platforms. Although a higher productivity can be realized through the utilization of an intermediate platform, such breaking of the cycle into two parts complicates the equipment and makes it considerably more expensive. The fact that two operators are also required to operate a dual hoist crane additionally and significantly raises the operating cost of the equipment.